Download Lipids

Chapter
Outline
Chapter 28
Lipids
Polar bears
have a large
reserve
of lipids.
Introduction to General, Organic, and Biochemistry, 10e
John Wiley & Sons, Inc
Morris Hein, Scott Pattison, and Susan Arena
Chapter
Outline
Course Outline
28.1 Lipids: Hydrophobic Molecules
28.2 Classification of Lipids
28.3 Simple Lipids
28.4 Fats in Metabolism
28.5 Compound Lipids
28.6 Steroids
28.7 Hydrophobic Lipids and Biology
Chapter 28 Summary
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Chapter
Outline
Lipids: Hydrophobic Molecules
Lipids are water-insoluble substances that have several
important biological functions that are related to how
lipids are classified.
Lipids interact weakly with water molecules because they
are composed primarily of nonpolar alkyl groups.
Lipids are classified as hydrophobic (“water fearing”) to
designate their inability to interact effectively with water
or their strong tendency to move away from water.
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Chapter
Outline
Lipids: Hydrophobic Molecules
Fatty acids are common components of lipids.
As fatty acids get larger, the water solubility of the
fatty acid decreases dramatically as shown on the
next slide . . .
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Chapter
Outline
Lipids: Hydrophobic Molecules
5
Chapter
Outline
Your Turn!
What characteristic do all lipids have in common?
6
Chapter
Outline
Your Turn!
What characteristic do all lipids have in common?
All lipids are insoluble in water.
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Chapter
Outline
Your Turn!
Which fatty acid would you predict to have the lowest
melting point?
O
CH3CH2CH2C
O
OH
CH3CH2CH2CH2CH2C
OH
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Chapter
Outline
Your Turn!
Which fatty acid would you predict to have the lowest
melting point?
O
CH 3CH 2CH 2C
o
m.p. = -7.9 C
O
OH
CH 3CH 2CH2CH2CH2C
OH
o
m.p. = -3.4 C
Both fatty acids are saturated. The fatty acid with the
fewest number of carbon atoms should have the
lowest melting point.
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Chapter
Outline
Classification of Lipids
Lipids molecules are relatively large and nonpolar. Yet,
within this broad description, lipid structures vary
markedly. Lipids can be classified in four categories
which recognize major structural similarities.
• Simple lipids
• Compound lipids
• Steroids
• Miscellaneous lipids
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Chapter
Outline
Simple Lipids
The simple lipids include fats, oils, and waxes. These
simple lipids are derivatives of lipid-like substances call
fatty acids.
Fatty acids are long-chain carboxylic acids (generally
greater than about 12 carbons) that have no solubility in
water.
The hydrophilic -COOH group is referred to as a polar
head and the hydrophobic hydrocarbon portion is
referred to as a nonpolar tail.
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Chapter
Outline
Simple Lipids
Fats and waxes are solids due to a higher composition of
saturated fatty acids while oils are liquids due to a
higher composition of unsaturated fatty acids.
Table 28.1 on the next slide lists properties of saturated
and unsaturated fatty acids separately. Notice how
unsaturated fatty acids generally have lower melting
points than saturated fatty acids.
Lipids prepared from fatty acids with lower melting points
tend to be liquids instead of solids at room temperature.
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Chapter
Outline
Simple Lipids
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Chapter
Outline
Simple Lipids
Unsaturated fatty acids exist as cis and trans isomers. The
cis isomer is more prevalent in nature.
The cis and trans isomers of oleic acid are shown on the
next slide.
Oleic acid is an unsaturated fatty acid. The cis isomer has a
bent structure which prevents close stacking resulting in
a compound that resists solidification.
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Chapter
Outline
Simple Lipids
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Chapter
Outline
Your Turn!
Draw the cis and trans isomers of palmitoleic acid.
CH3(CH2)5CH=CH(CH2)7COOH.
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Chapter
Outline
Your Turn!
Draw the cis and trans isomers of palmitoleic acid.
CH3(CH2)5CH=CH(CH2)7COOH.
H
C
C
H
trans-isomer
H
H
C
C
cis-isomer
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Chapter
Outline
Simple Lipids
Certain fatty acids, as well as other lipids, are biochemical
precursors of several classes of hormones.
For example, ω-6 (omega-6) and ω-3 fatty acids are used
to make hormones, the most common of which are the
eicosanoids.
These hormones are derived from fatty acids with 20
carbon atoms, either the ω-6 (arachidonic acid) or the
ω-3 (eicosapentaenoic acid).
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Chapter
Outline
Simple Lipids
Omega (ω) is the last letter in the Greek alphabet.
Correspondingly, the last carbon atom in a carbon chain
of a compound is often referred to as the omega carbon.
In reference to unsaturated carboxylic acids, omega plus a
number (e.g., ω-3) indicates the location of the first
carbon–carbon double bond, counting from the omega
carbon.
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Chapter
Outline
Simple Lipids
So for example arachidonic acid is an ω-6 fatty acid
because the first double bond from the last carbon atom
is on the sixth carbon atom.
arachidonic acid
COOH
5
6
4
3
2
CH 3
1
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Chapter
Outline
Simple Lipids
Examples of eicosanoids are:
•
•
•
•
Thromboxanes
Prostacyclins
Prostaglandins
Leukotrienenes
The synthesis of eicosanoids from arachidonic acid is
shown on the next slide . . .
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Chapter
Outline
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Chapter
Outline
Simple Lipids
Eicosanoids are hormones and coordinate various cellular
responses.
• Some are involved with blood clotting as they can cause
platelet aggregation while others trigger an increase in
body temperature.
• Some eicosanoids coordinate HCl and mucous secretion
by the stomach lining and constriction of the bronchial
tubes in the lungs.
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Chapter
Outline
Simple Lipids
• Some eicosanoids stimulate and attract white cells,
while other eicosanoids cause the white cells to
disperse.
• Eicosanoids can also cause vasodilation as well as
vasoconstriction.
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Chapter
Outline
Simple Lipids
Many drugs control one or more of the physiological
effects produced by the eicosanoids.
For example, aspirin is a nonsteroidal anti-inflammatory
drug (and NSAID) that blocks the oxidation of
arachidonic acid which in turn prevents the formation
of prostaglandins and thromboxanes.
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Chapter
Outline
Simple Lipids
Fats and oils are triester derivatives of glycerol and three
fatty acid molecules as shown below.
Because there are three ester groups per glycerol, these
lipids are called triacylglycerols or triglycerides. The
fatty acids have carbon chains with 14-18 carbon atoms.
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Chapter
Outline
Simple Lipids
Triglycerides are prepared by the reaction of a glycerol
molecule and three fatty acid molecules. The fatty acid
molecules can be saturated or unsaturated.
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Chapter
Outline
Simple Lipids
Waxes are esters of high-molar-mass fatty acids and highmolar-mass alcohols.
They have the general formula shown below in which the
alcohol (ROH) contributes up to about 30 carbons.
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Chapter
Outline
Simple Lipids
Waxes are very large molecules with almost no polar
groups. They represent one of the most hydrophobic
classes of lipids.
Their extreme water insolubility allows waxes to serve a
protective function. Leaves, feathers, fruit, and fur are
often naturally coated with wax.
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Chapter
Outline
Simple Lipids
An important biological function of a wax is to act as a
protective coating. The “shine” on these leaves is due
to a thick protective wax coating.
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Chapter
Outline
Your Turn!
Write the formula for a wax formed from palmitic acid
[CH3(CH2)14COOH] and 1-hexacosanol
[CH3(CH2)24CH2OH].
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Chapter
Outline
Your Turn!
Write the formula for a wax formed from palmitic acid
[CH3(CH2)14COOH] and 1-hexacosanol
[CH3(CH2)24CH2OH].
palmitic acid portion
O
O
1-hexacosanol portion
As with all waxes, this wax is an ester.
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Chapter
Outline
Your Turn!
What is the correct classification of the fatty acid shown
below?
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
• ω-3
• ω-6
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Chapter
Outline
Your Turn!
What is the correct classification of the fatty acid shown
below?
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
• ω-3
• ω-6
The double bond is attached to the sixth carbon from the
end of the molecule furthest from the carboxyl group.
This is an ω-6 fatty acid.
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Chapter
Outline
Fats in Metabolism
Fats are an important food source for humans and
normally account for about 25–50% of our caloric
intake. Fats are an especially good source of metabolic
energy.
Most metabolic energy is derived from carbon oxidation.
When oxidized to carbon dioxide and water, fats supply
about 40 kJ per gram (9.5 kcal/g), which is more than
twice the amount obtained from carbohydrates or
proteins.
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Chapter
Outline
Fats in Metabolism
Fats are what our bodies prefer when storing energy
reserves. These reserves are in the form of
triacylglycereols in fatty tissue.
On average, this tissue stores about two to three weeks’
worth of energy.
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Chapter
Outline
Your Turn!
The carbohydrate glucose (C6H12O6) and the fatty acid
capric acid (C9H19COOH) have similar molecular
weights. Recall that most metabolic energy is derived
from carbon oxidation. Using these two molecules as
examples, why does fat supply more energy than an
equivalent amount of carbohydrate?
37
Chapter
Outline
Your Turn!
The carbohydrate glucose (C6H12O6) and the fatty acid
capric acid (C9H19COOH) have similar molecular
weights. Recall that most metabolic energy is derived
from carbon oxidation. Using these two molecules as
examples, why does fat supply more energy than an
equivalent amount of carbohydrate?
Fats are about 75% carbon by mass as compared to about
40% for carbohydrates. In addition fats have a lower
degree of carbon oxidation when compared to
carbohydrates which results in higher energy yields
38
when fats are oxidized.
Chapter
Outline
Compound Lipids
There are three broad categories of compound lipids.
• Phospholipids
• Sphingolipids
• Glycolipids
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Chapter
Outline
Compound Lipids
The phospholipids are a group of compounds that yield
one or more fatty acid molecules, a phosphate group,
and usually a nitrogenous base upon hydrolysis.
In contrast to the triacylglycerols, phospholipids have a
hydrophilic end that interacts with water.
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Chapter
Outline
Compound Lipids
Phospholipids are one of the most important membrane
components.
They are also involved in the metabolism of other lipids
and nonlipids.
Three categories of phospholipids are phosphatidic acids,
phosphatidylcholines, and phosphatidylethanolamines.
41
Chapter
Outline
Compound Lipids
Phosphatidic acids are glyceryl esters of fatty acids and
phosphoric acid. The phosphatidic acids are important
intermediates in the synthesis of triacylglycerols and
other phospholipids.
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Chapter
Outline
Compound Lipids
Phosphatidylcholines (lecithins) are glyceryl esters of fatty
acids, phosphoric acid, and choline.
43
Chapter
Outline
Compound Lipids
The single most important biological function for
phosphatidylcholine is as a membrane component which
makes up between 10 and 20% of many membranes.
44
Chapter
Outline
Compound Lipids
Another important constituent of biological membranes is
the phosphatidylethanolamines (cephalins). These
lipids are glyceryl esters of fatty acids, phosphoric acid,
and ethanolamine (HOCH2CH2NH2).
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Chapter
Outline
Compound Lipids
Sphingolipids are another type of compound lipid that,
when hydrolyzed, yield:
• a hydrophilic group (either phosphate and choline or a
carbohydrate)
• a long-chain fatty acid (18–26 carbons)
• sphingosine (an unsaturated amino alcohol)
46
Chapter
Outline
Compound Lipids
The similarities between sphingosine and glycerol are
shown in the structures below.
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Chapter
Outline
Compound Lipids
Sphingolipids are common membrane components because
they have both hydrophobic and hydrophilic character.
Sphingomyelins are found in the myelin sheath membranes
that surround nerves.
48
Chapter
Outline
Compound Lipids
Glycolipids are a third type of compound lipid. These
compounds that contain a carbohydrate group.
The two most important classes of glycolipids are
cerebrosides and gangliosides. These substances are
found mainly in cell membranes of nerve and brain
tissue.
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Chapter
Outline
Compound Lipids
A cerebroside may contain either D-galactose or Dglucose. The following formula of a galactocerebroside
shows the typical structure of cerebrosides.
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Chapter
Outline
Your Turn!
What is the classification of the following lipid?
51
Chapter
Outline
Your Turn!
What is the classification of the following lipid?
This lipid is an example of a phospholipid and more
specifically a phosphatidylethanolamine.
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Chapter
Outline
Your Turn!
What is the classification of the following lipid?
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Chapter
Outline
Your Turn!
What is the classification of the following lipid?
This molecule
is a
glycolipid.
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Chapter
Outline
Steroids
Steroids are compounds that have the steroid nucleus,
which consists of four fused carbocyclic rings. This
nucleus contains 17 carbon atoms in one five-membered
ring and three six-membered rings.
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Chapter
Outline
Steroids
Modifications of this nucleus that occur in the various
steroid compounds include, for example, added side
alkyl chains, hydroxyl groups, carbonyl groups, and ring
double bonds.
Steroids are closely related in structure but are highly
diverse in function . . .
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Chapter
Outline
Steroids
For example:
• Cholesterol, the most abundant steroid in the body, is
widely distributed in all cells and serves as a major
membrane component.
• Bile salts aid in the digestion of fats.
• Ergosterol, a yeast steroid, is converted to vitamin D by
ultraviolet radiation.
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Chapter
Outline
Steroids
• Digitalis and related substances called cardiac glycosides
are potent heart drugs.
• The adrenal cortex hormones are involved in
metabolism.
• The sex hormones.
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Chapter
Outline
Steroids
Cholesterol is the parent steroid compound from which the
steroid hormones are synthesized. In this process
cholesterol is converted to progesterone, a compound
that helps control the menstrual cycle and pregnancy.
This hormone is also the parent compound from which
testosterone and the adrenal corticosteroids are
produced.
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Chapter
Outline
Steroids
Cholesterol is also used to build cell membranes, many of
which contain about 25% by mass of this steroid.
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Chapter
Outline
Your Turn!
Identify the functional groups in cortisone.
CH 2OH
CH 3
C
O
O
CH 3
O
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Chapter
Outline
Your Turn!
Identify the functional groups in cortisone.
hydroxyl group
CH 2OH
carbonyl group
CH 3
C
O carbonyl group
O
CH 3
O
carbonyl group
carbon-carbon
double bond
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Chapter
Outline
Hydrophobic Lipids and Biology
The hydrophobic nature of lipids has many important
biological consequences. The water insolubility of
lipids results in:
• lipid aggregation that causes atherosclerosis
• lipid aggregation that forms biological membranes
A lipid is in a hostile environment when it is surrounded
by water. Lipid molecules aggregate to minimize their
contact with water when in water.
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Chapter
Outline
Hydrophobic Lipids and Biology
The hydrophilic part of lipid molecules is attracted to water
and forms an interface with it, but the hydrophobic part
distances itself from water molecules.
Complex lipids such as phospholipids and sphingolipids
have two hydrophobic alkyl groups.
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Chapter
Outline
Hydrophobic Lipids and Biology
Complex lipids form liposomes in aqueous mixtures.
Liposomes are bounded by two layers of lipid.
The hydrophobic alkyl chains are covered by hydrophilic
groups on both the liposome’s inside and outside.
Liposomes have a water core as shown below.
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Chapter
Outline
Hydrophobic Lipids and Biology
Atherosclerosis is a metabolic disease that leads to
deposits of cholesterol and other lipids on the inner
walls of the arteries. Blood pressure increases as the
heart works harder to pump sufficient blood through the
narrowed passages, which may eventually lead to heart
attack.
Plaque formation begins because of a lipid’s natural
tendency to aggregate.
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Chapter
Outline
Hydrophobic Lipids and Biology
Improper transport of cholesterol through the blood
contributes to atherosclerosis.
Cholesterol (and other lipids) must be packaged for
transport because lipids aggregate in the aqueous
bloodstream.
Lipids are packaged in particles called lipoproteins.
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Chapter
Outline
Hydrophobic Lipids and Biology
The main types of lipoproteins are VLDL (very-lowdensity lipoprotein), LDL (low-density lipoprotein)
and HDL (high-density lipoprotein).
The lipid distribution system through the bloodstream
using these lipoproteins is shown on the following
slide . . .
68
Chapter
Outline
Hydrophobic Lipids and Biology
69
Chapter
Outline
Hydrophobic Lipids and Biology
Biological membranes are thin, semipermeable cellular
barriers. The general function of these barriers is to
exclude dangerous chemicals from the cell while
allowing nutrients to enter.
Because almost all the dangerous chemicals, nutrients, and
special molecules are water soluble, the membranes can
act as effective barriers only if they impede the
movement of hydrophilic (water-soluble) molecules.
70
Chapter
Outline
Hydrophobic Lipids and Biology
The hydrophobic interior of the membrane provides the
necessary barrier while the hydrophilic exterior
interacts with the aqueous environment.
71
Chapter
Outline
Hydrophobic Lipids and Biology
Membrane lipids naturally aggregate to form lipid bilayers.
A lipid bilayer is composed of two adjoining layers of
lipid molecules aligned so that their hydrophobic
portions form the bilayer interior while their hydrophilic
portions form the bilayer exterior.
72
Chapter
Outline
Hydrophobic Lipids and Biology
All known cells in today’s world need a membrane that is
more complicated than a simple lipid bilayer.
A membrane must function as more than just a barrier.
Tasks such as passing molecules from one side of a
bilayer to the other are an essential part of life.
Proteins in the fluid bilayer solve this dilemma. They
allow specific molecular transport through the
hydrophobic interior.
73
Chapter
Outline
Hydrophobic Lipids and Biology
If the protein helps transport without using energy, the
process is called facilitated diffusion.
Energy-requiring transport is termed active transport.
74
Chapter
Outline
Hydrophobic Lipids and Biology
A complete cellular membrane must have both lipid and
protein. A typical membrane includes about 60%
protein, 25% phospholipid, 10% cholesterol, and 5%
sphingolipid.
The fluid lipid bilayer is studded with many solid
proteins. The proteins form a random pattern on the
outer surface of the oily lipid. This general membrane
is called the fluid-mosaic model which is shown on the
next slide . . .
75
Chapter
Outline
Hydrophobic Lipids and Biology
76
Chapter
Outline
Hydrophobic Lipids and Biology
Which statement best describes the composition of a
biological membrane?
•
•
•
•
Hydrophobic exterior and hydrophilic interior
Hydrophilic exterior and hydrophobic interior
Hydrophilic exterior and interior
Hydrophobic exterior and interior
77
Chapter
Outline
Hydrophobic Lipids and Biology
Which statement best describes the composition of a
biological membrane?
•
•
•
•
Hydrophobic exterior and hydrophilic interior
Hydrophilic exterior and hydrophobic interior
Hydrophilic exterior and interior
Hydrophobic exterior and interior
Biological membranes have hydrophilic exteriors and
hydrophilic interiors.
78
Chapter
Outline
Chapter 28 Summary
• Lipids are water-insoluble, oily, or greasy biochemicals.
All lipids are relatively large and nonpolar, yet lipid
structures may differ markedly.
• Simple lipids are esters of fatty acids and alcohols.
• Fats and oils are esters of fatty acids and glycerol.
• Waxes are esters of fatty acids and high-molar-mass
alcohols.
79
Chapter
Outline
Chapter 28 Summary
• Compound lipids are composed of fatty acid esters and
other components.
• Phospholipids yield a phosphate and a nitrogencontaining base as well as glycerol and fatty acids upon
hydrolysis.
• Sphingolipids yield an unsaturated amino alcohol
(sphingosine) and a carbohydrate or phosphate and
nitrogen base in addition to a fatty acid upon hydrolysis.
80
Chapter
Outline
Chapter 28 Summary
• Glycolipids yield sphingosine and a carbohydrate as well
as a fatty acid upon hydrolysis.
• Steroids posses the steroid nucleus containing 17 carbon
atoms.
• Miscellaneous lipids include lipoproteins.
81
Chapter
Outline
Chapter 28 Summary
• Fatty acids, which form part of most lipids, are
carboxylic acids with long, hydrophobic carbon chains.
• Common fatty acids are straight-chain compounds with
an even number of carbons.
• Unsaturated (double bond–containing) fatty acids are
commonly cis isomers.
• Omega-3 and omega-6 fatty acids are essential in human
diets. The eicosanoids are formed by oxidizing either
82
omega-3 or omega-6 fatty acids.
Chapter
Outline
Chapter 28 Summary
• Fats and oils are esters of glycerol and long carbon chain
fatty acids.
• Waxes are esters of fatty acids and a high-molar-mass
alcohol (up to about 30 carbons long).
• Fats are a rich source of metabolic energy. They contain
a high percentage of carbon, whose oxidation releases
energy.
• Fats are our primary energy-storage molecules.
83
Chapter
Outline
Chapter 28 Summary
• The phospholipids are a group of compounds that yield
one or more fatty acid molecules, a phosphate group,
and a nitrogenous base upon hydrolysis.
• Phospholipids combine hydrophobic and hydrophilic
properties, essential features of membrane lipids.
• Sphingolipids are compounds that, when hydrolyzed,
yield a hydrophilic group, a fatty acid, and sphingosine.
84
Chapter
Outline
Chapter 28 Summary
• Glycolipids are sphingolipids that contain carbohydrates.
• Steroids are compounds that have the steroid nucleus.
• The combined hydrophobic and hydrophilic character of
some lipids causes them to aggregate into specific
structures when placed in water.
• Atherosclerosis is a metabolic disease in which fatty
material (plaque) is deposited on the inner walls of
arteries.
85
Chapter
Outline
Chapter 28 Summary
• Improper transport of cholesterol through the blood
system contributes to atherosclerosis. Lipoproteins
(specific protein-lipid aggregates) transport lipids
through the blood.
• A lipid bilayer is composed of two adjoining layers of
compound lipid molecules.
• Proteins aid passage of molecules through a lipid
bilayer.
86